The present invention relates to an electron scanning microscope for carrying out dimensional measurement of a micro-pattern formed on a semiconductor substrate, a method of evaluating a resolution of a scanning electron microscope, and a sample for valuating a resolution of a scanning electron microscope, and in particular to a scanning electron microscope incorporating a function of evaluating a resolution of a scanning electron microscope from a picked-up image.
In a semiconductor manufacturing process, there have been demanded apparatuses for measuring dimensions with a higher degree of accuracy as the micro-patterns have been more and more fine. There has been known a scanning electro microscope for measuring a pattern width (a length measuring SEM (scanning electron microscope), or a CD (critical dimension) SEM), which are capable of picking up an image thereof with a magnification of one to five hundreds of thousands (100,000-500,000) as a dimension measuring tool for measurement of a micro-pattern having a size in the order of several ten nanometers.
The demands for measuring accuracy of these apparatuses include not only enhancing the measuring accuracy of the individual apparatus but also reducing differences among measured dimensions of several apparatuses installed on a production line and as well as reducing variations in measured dimensions which are caused by aging (or deteriorating with age) of the apparatus.
Of many factors for causing differences among measured dimensions of several apparatuses and for occurrence of variations in measured dimensions due to aging of the apparatuses, there may be exemplified differences and variation in resolution caused by differences among beam sizes and/or variation in the beam size due to aging. However, it is difficult to directly measure a size of an electron beam. Thus, in a scanning electron microscope, there has been used such a process that index values of resolution are measured from SEM images picked up by respective apparatuses, and differences among the beam sizes are evaluated by comparative evaluation of the index values.
As a specific example of a technique for measuring a resolution, U.S. Pat. No. 6,545,275 (Patent Document 1) and Metrics of resolution and performance for CD-SEMs by D. C. Joy et al, Metrology, Inspection, and Process Control for Microlithography XIV, page 108 (Nonpatent Document 1) propose, as examples thereof, a method in which an image is picked up from a sample prepared by depositing gold particles on a silicon substrate, and frequencies are analyzed through Fourier transformation of the picked-up image in order to calculate an index value of resolution. Further, U.S. Pat. No. 5,969,273 (Patent Document 2) and Modeling and Experimental Aspects of Apparatus Beam Width as an Edge Resolution Measure, C. Archie et al, Metrology, Inspection, and Process Control for Microlithography XIII, page 669 (Nonpatent document 2) propose such a technique that an image is picked up from a pattern formed on a substrate so as to measure a width corresponding to a pattern edge part in order to calculate an index value of resolution. Furthermore, JP-A-2005-268231 (Patent Document 3) and Contrast-to-gradient method for the evaluation of image resolution taking account of random noise in scanning electron microscopy, T. Ishitani et. al, J. Electron Microscopy 53(3) page 245 (Nonpatent Document 3) propose such a technique that a plurality of partial resolutions is obtained from respective partial zones in a picked-up image, and an average of partial resolutions over the entire image is calculated in order to calculate an index value of resolution.
In a scanning electron microscope apparatus for measuring dimensions of a pattern, a conventional resolution measuring process in which a picked-up image is used comprises the steps of (A-1) acquiring a picked-up image from a sample which is silicon substrate deposited thereon with gold or a porous silicon substrate, and (B-1) subjecting the picked-up image to Fast Fourier Transformation in order to analyze frequencies so as to calculate an index value of resolution. Further, another conventional resolution measuring process comprises the steps of (A-2) acquiring an image picked up from a pattern formed on a substrate, and (B-2) measuring a width corresponding to an edge part of the pattern from the picked-up image so as to calculate an index value of resolution.
A secondary electron image obtained by the scanning electron microscope, is in general exhibited by a convolution integration of a f(x, y) and g(s,t), where f(x,y) is a signal determined by a material of a sample and a pattern shape, and g(s,t) is a shape of an electron beam irradiated onto the sample. That is, in order to measure a size of an electron beam from a secondary electron image, it is required to take into consideration an influence caused by the signal f (x, y) which is determined by a sample and which is included in the image.
Estimating that the measurement of a resolution is carried out with the use of a dedicated sample, it is desirable for the sample to have one and the same pattern, one and the same pattern sectional shape and one and the same pattern distribution everywhere on the sample, even though it is not required to consider a variation in the signal due to a material quality. However, it is impossible to prepare such a sample, and the following problem will be caused.
Since the sample used in (A-1) has such a feature that analogous patterns each having a size of several ten nanometers are distributed in random over the entire surface of the sample, if an image having many patterns can be obtained, it is expected to calculate an index value of resolution with respect to an averaged value of the signals (x, y). However, if the distribution densities, the averaged sizes or pattern sizes of the analogous patterns, are uneven, or if the sectional shapes vary thereamong, the averaged value of the signals (x,y) will be of course, changed, and accordingly, the sample should be prepared by controlling these items in order to decrease the dependency upon the individual sample characteristics. Similarly, even with a pattern formed on a substrate used in (A-2), the signals f(x,y) are different from one another, and accordingly, the index value of resolution depends upon the individual sample characteristics.
Further, even with respect to a resolution evaluating algorithm, in the technique used in (B-1), if a pattern distribution on a sample becomes different, an index value of resolution obtained by calculation has a different characteristic and therefore dependency upon a pattern does not become small. In the technique used in (B-2), if a pattern roughness becomes different in an image zone used for calculation of a resolution, the index value of resolution will change, and accordingly, the pattern dependency is also not negligible.
In view of the above-mentioned matters regarding the resolution monitor, a resolution problem inevitably has such a task that the preparation of a sample and the utilization of a measurement algorithm, which are capable of reducing the pattern dependency of an index value of resolution to be measured are required for precisely measuring a variation in size of an electron beam.